This invention relates to a wave guide type optical device as an optical integrated circuit having formed thereon lenses and gratings having particularly high efficiency, and to fabrication method thereof.
Optical components for use in the fields of optical communication systems and for optical data processing have been constituted conventional by combining mechanically bulk components such as lenses, prisms and gratings. Therefore, the optical components of this kind have a large outer dimension and cannot meet with demands for miniaturization. Moreover, the cost of production is high and since they are assembled by mechanical combination, stability as well as reliability drop in the course of extended period of time. For this reason, the concept of optical IC has been introduced recently. According to this concept, a plurality of elements are integrated on one substrate, and drastic reduction of the size and cost of production of optical components has been examined. In other words, light emission/reception elements and wave guide type (thin film type) lenses and gratings are integrated on one substrate to constitute an optical component.
To form the optical devices such as the wave guide type lenses and grating of the type described above, two means have been employed to date. As described, for example, in Japanese Patent Laid-Open No. 113903/1983, the first means employs ion implantation, thermal diffusion, proton exchange or other techniques and forms a region having a higher refractive index than that of a light guide path. As discussed in R. Ulrich et al., "Geometrical Optics in Thin Film Light Guides; Applied Optics", 10, 9(1971), the second means forms a loading layer having a different refractive index on an optical wave guide.
With reference to the prior art technique described above, the first means involves the problem that since the region having a different refractive index is formed by diffusion or ion implantation, the distribution of refractive index cannot be controlled easily, thermal stability is low and efficiency of light wave is low. The second means is not considered sufficiently the difference of thermal and mechanical characteristics between the optical wave guide path and the optical wave control layer formed on the former. When the loading layer is formed directly on the optical wave guide, therefore, internal stress develops inside the film due to the difference of properties at the time of film formation and crack and peel of the control layer develop.